Rolling Sum with a Month Prompt

HI GUys,

I have a requirement where  I need Rolling sum for 12 months. It should always take 12 months irrespective of the year.

So I created three variables:-

First for Denominator =RunningSum([Total Readings];([Machine]))+RunningSum(Previous([Total Readings];11);([Machine]))

Second for Numerator = =RunningSum([Good Readings];([Machine]))+RunningSum(Previous([Good Readings];11);([Machine]))

Third and Final =[Numerator]*100/[Denominator]

The result comes fine but here is the glitch. User wants to see rolling average against Current month only so I created a Prompt for Month so when WEBI is refreshed they can enter Month Value and run the webi. But it does not take previous 11 months in to consideration.

How can I achieve that if user enters Month as February for year 2014 The Rolling sum should  take All values from March 2013 till February 2014 into calculation.

Please help.

Regards

AS